Have you ever made an airline reservation and been told the only available seats are those in the middle? What you probably weren’t told is that airlines hold a number of prime seats for last-minute bookings to elite customers. When those seats aren’t sold, they’re released about 24 hours prior to departure.
To take advantage of this, visit your airline’s website and look for the “check-in” link. There you will see what (newly released) seats are available, and you can change yours if necessary.
If that doesn’t work, use the check-in kiosk when you get to the airport. It offers the same option.